1. Field of the Invention
The invention relates to a video camera apparatus and, more particularly, to the elimination of flickers which are generated when an object is photographed under a fluorescent lamp.
2. Description of the Prior Art
For instance, when an object is video photographed by a video camera of the NTSC system under a fluorescent lamp which is lit on by a commercially available power source of a frequency of 50 Hz, flickers of a frequency of 20 Hz are generated due to the relation between the field frequency and the lighting frequency of the fluorescent lamp. To eliminate such flicker components, there is proposed a flicker cancelling circuit for detecting the flicker components of a frequency of 20 Hz in an image pickup signal and for variably changing a gain of an AGC so as to cancel the flicker components.
That is, FIG. 1 shows an example of a flicker cancelling circuit of a conventional video camera. In FIG. 1, an object image transmitted through a lens 51 is formed onto a CCD image pickup device 53 through an iris 52. An output of the CCD image pickup device 53 is supplied to an AGC amplifier 54. An output of the AGC amplifier 54 is supplied to a signal processing circuit 55 and to a detecting circuit 56.
The level of image pickup signal is detected by the detecting circuit 56. An output of the detecting circuit 56 is supplied to a comparator 57 and to a band pass filter 58 of a frequency of 20 Hz. A predetermined reference level is set into the comparator 57. The image pickup signal level is compared with the predetermined reference level by the comparator 57. An output of the comparator 57 is supplied to an adding circuit 60 through a loop filter 59. An output of the adding circuit 60 is supplied to the AGC amplifier 54. The gain of the AGC amplifier 54 is controlled in accordance with the output of the adding circuit 60. The output of the detecting circuit 56 is compared with the predetermined reference level by the comparator 57. The image pickup signal level is controlled so as to become the predetermined reference level by a loop for controlling the gain of the AGC amplifier 54 by an output of the comparator 57.
The component of a frequency of 20 Hz in the image pickup signal is detected by the band pass filter 58. An output of the band pass filter 58 is supplied to the adding circuit 60 through a filter 61 and a gain amplifier 62. The flicker components of the frequency of 20 Hz in the output of the detecting circuit 56 are extracted by the band pass filter 58. The flicker components of the frequency of 20 Hz are cancelled by a loop for controlling the gain of the AGC amplifier 54 by the flicker components.
That is, in the above circuit, as shown in FIG. 2, AGC loop gain characteristics such as to increase the gain of the portion of the frequency of 20 Hz are set. In FIG. 2, an axis of abscissa denotes the frequency and an axis of ordinate indicates the gain. The flicker components of the frequency of 20 Hz are cancelled by setting such loop gain characteristics.
In the flicker cancelling circuit of the conventional video camera shown in FIG. 1, however, the exposure control loop for controlling such that the image pickup signal level is set to the predetermined reference level and the flicker cancelling loop for controlling such that the flicker components of the frequency of 20 Hz are extracted and the flicker components are cancelled are constructed by using the same detecting circuit 56. Therefore, for instance, when weight amounts of an exposure detection area in the central portion and an exposure detection area in the peripheral portion are changed at the time of a backlight, there occurs a problem such that the flicker cancelling characteristics also change.
Namely, in the case of the backlight, the iris is closed and the gain of the AGC amplifier is reduced, so that the object becomes dark. As shown in FIG. 3, therefore, exposure detection areas A1 and A2 are provided for the central portion and peripheral portion of the picture plane, respectively. The weight of the image pickup signal level of the exposure detection area A1 in the central portion of the picture plane is set to a large value in the backlight state, thereby controlling so as to execute the center significance photometric operation. Consequently, it is possible to prevent that the object becomes dark. In the case where the exposure control loop and the flicker cancelling loop are constructed by using the same detecting circuit 56, there occurs a problem such that the flicker cancelling characteristics also change when the weight amounts of the exposure detection area A1 of the central portion and the exposure detection area A2 of the peripheral portion are changed in accordance with whether the light is a back light or a front light.